1. Field of the Invention
The present invention relates generally to magnet systems, and in particular to a non-persistent switch for use with a superconducting magnet.
2. Discussion of the Related Art
As is well known, a magnet can be made superconducting by placing it in an extremely cold environment, such as by enclosing it in a cryostat or pressure vessel containing liquid helium or other cryogen. The extreme cold reduces the resistance in the magnet coils to negligible levels. After the power source that is initially connected to the coil is removed, the current will continue to flow through the magnet coils relatively unimpeded by the negligible resistance, thereby maintaining a magnetic field.
To maintain current flow in the magnet coils after removal of power, it is typically necessary to complete the electric circuit within the cryogenic environment with a superconducting switch that is connected in parallel with the power supply and the magnet coils. The superconducting switch generally consists of a superconducting conductor, which when driven into the non-superconducting or normal state, has sufficient resistance so that current from the power supply will essentially flow through the magnet coils during “ramp-up.” When the desired magnetic field current is achieved, the switch is returned to its superconducting state and the magnet current commutates out of the power supply and through the switch when the power supply is ramped down. The magnet is now in what is referred to as “persistent mode.”
There are four characteristics that a superconducting switch typically exhibits. One, it must be capable of easily and quickly being transformed (switched) from the superconducting state to the normal state, and vice versa. Three ways this can be done are: a) thermally—by heating the superconducting material above its transition temperature; b) magnetically—by applying a magnetic field greater than the critical field of the material; or c) electrically—by raising the current in the material above its critical current. The thermal method is the most common. Two, it must have a high enough resistance in its normal state such that current flow through the switch during ramp-up is negligible so that excessive heating in the cryogen environment is not produced. Three, the switch must be stable. That is, other than during a desired transition phase, it must not transition from the superconducting to normal state. Four, it must be capable of carrying the same high currents as the magnet coils.
Conventional persistent switches of the thermal type operate by heating the superconducting material to a temperature above its superconducting critical temperature. One known thermal persistent switch includes a resistive wire wound about the superconducting wire. Normalization of the superconducting material of the switch is effected by applying electrical current to the resistive wire, thereby heating the superconducting material to above its critical temperature. One of the challenges in designing a superconducting switch is to balance the conflicting requirements of minimizing transition time between a superconducting state and a resistive state, and the need for low heat output to minimize cryogen boil-off.